Contents
Section 1..............................................................................3 Introduction .........................................................................3 Section 2..............................................................................4 Applications.........................................................................4 Section 3..............................................................................6 The Vacuum Forming Process ...........................................6 Clamping .............................................................................7 Heating.................................................................................7 Sheet Level...........................................................................8 Pre-stretch ( Bubble) ...........................................................8 Vacuum ...............................................................................8 Plug Assist .............................................................................8 Cooling and Release..........................................................8 Section 4............................................................................10 Plastic Materials and Their Characteristics.....................10 Acrylonitrile Butadiene Styrene– (ABS) ...........................13 Acrylic - PMMA – (Perspex, Oroglas, Plexiglas).............14 Co-Polyester – (PETG / VIVAK) .........................................15 Polystyrene– Polyphenylethene (H.I.P.S / BEXTRENE) ...16 Polycarbonate – (P.C. / LEXAN/ MAKROLON)...............17 Polypropylene – (PP).........................................................18 Polyethelene – (PE, HDPE, LDPE, PE FOAM) ...................19 Polyvinylchloride – (PVC) .................................................20 Section 5............................................................................21 Mould and Mould Design ................................................21 Mould Materials.................................................................22 Male and Female Moulds ................................................28 Baseboards and Mounting ..............................................29 Draught Angles / Tapers...................................................30
Venting ...............................................................................30 Shrinkage and Mould Release ........................................31 Undercuts, Split and Multi Impression Moulds ................32 Mould Cooling...................................................................33 Plug Assist Design ..............................................................33 Webbing/ Chill Marks/ Thinning.......................................35 Webbing ............................................................................35 Chill Marks ..........................................................................36 Thinning ..............................................................................36 Section 6............................................................................37 Finishing and Trimming......................................................37 Considerations...................................................................38 Heated knife/ Scalpel.......................................................39 Vertical Bandsaw ..............................................................40 Horizontal Bandsaw (Fig 6.4)............................................40 Rollerpress/ Steel Rule Dies (g 6.5) .................................41 Guillotine ............................................................................42 Clicker / Punch Press.........................................................42 Hand Held Air Powered Router .......................................42 CNC Routers - 3, 4 and 5 axis...........................................43 Circular spindle saw..........................................................43 Circular Cutter mounted from under table. ..................44 Section 7............................................................................45 Trouble Shooting Guide....................................................45 Section 8............................................................................50 Supplier Guide...................................................................50 Section 9............................................................................53 Further Information............................................................53
Section 1
Introduction
Thermoforming is one of the oldest and most common methods of processing plastic materials. Thermoformed plastic products are all around us and play a major part in our daily lives. It is a very versatile process used to manufacture a wide range of products from simple packaging trays to high impact aircraft cockpit covers. It is also used extensively to make design prototypes of products to be produced by other processes.
not impart the practical experience and skill that any user will eventually attain. We hope it assists in taking the ‘guess work’ out of the process and proves useful as a means of technical and engineering support. In the following sections we have provided a list of the various examples of applications for thermoformed parts along with an insight into the forming process and techniques. A plastics section follows in which we examine materials and their characteristics. Information on tooling, trimming and nishing, a trouble shooting guide and recommended suppliers list concludes the manual with a glossary of key words to aid navigation through the manual. If you are new to the process and have just purchased a Formech machine we trust you nd this manual a benecial aid in getting started and wish you success in your future business.
The process, however, is basically the same in each case. In its simplest form thermoforming is the heating of a plastic sheet which is then draped over a mould whilst a vacuum is applied. The moulding is then allowed to cool before it is ejected from the mould using a reverse pressure facility. Thermoforming covers all processes which involve heat to shape polymers. However, in this manual we will concentrate specically on the ‘vacuum forming process’ which applies to the range of machines Formech supply. Vacuum forming has generally been promoted as a ‘dark art’ and best left to companies with sophisticated processing equipment who are able to supply the facility and service. However with their range of compact, easy to use machines Formech have, over the years, endeavoured to introduce vacuum forming to the ‘masses’ and it is some testament that there are now over six thousand machines worldwide, many supplied to those looking to start vacuum forming for the rst time. In this manual we hope to provide an insight into this adaptable process. However, it is only a guide and can-
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Section 2
Applications
Below we have allocated sub-headings to a wide range of industries using vacuum forming for a multitude of applications. The list is fairly comprehensive and although there are many other potential applications we have attempted to highlight the most popular. You can nd photographic examples of a selection of vacuum formed products in the galleries contained in the ‘support’ section of our web page. Aeronautical Manufacturers Interior Trim Panels, Covers and Cowlings Internal sections for NASA Space Shuttle Agricultural Suppliers Seed Trays, Flower Tubs, Animal Containers, Clear Growing Domes Calf Milking Receptacles, Machines Parts, Lawnmower Enclosures and Covers
Utility Shelves, Liners, Seat Backs, Door Innerliners and Dash Surrounds Windshields, Motorcycle Windshields, Golf Cart Shrouds, Seats and Trays Tractor Shrouds & Door Fascia, Camper Hardtops and Interior Components Building and Construction Industry Drainpipe Anti Dripttings Roof Lights, Internal Door Liners, PVC Door Panels Producing Moulds for Concrete Paving Stones and Special Bricks Moulded Features for Ceilings, Fireplaces, Porches and other items. Boat Building industry Boat Hulls, Covers and Hatches Electrical Enclosures, Dashboards Chocolate industry Manufacture of Chocolate Moulds for Specialised Chocolates and Easter Eggs etc. and Packaging
Architectural Model Makers Production of Miniature Parts for Architectural Models Prototypes.
Computer Industry Manufacture of Screen Surrounds Soft Transparent Keyboard Covers Enclosures and Ancillary Equipment
Automotive and Vehicular Industry Wheel Hub Covers, Ski-Boxes and Storage Racks Wind Tunnel Models, Parts for All Terrain Vehicles Truck Cab Door Interiors, Wind and Rain Deectors Scooter Shrouds, Mudguards, Bumpers and Protective Panels Battery and Electronic Housings, Prototype and Development work
Design Industry Production of prototypes and Pre - Production Runs Prototype Cocepts for other Plastic Processes
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Section 2/Applications Education Training Aids for Students Studying Polymers and Plastic Processing. Electronics Industry Manufacturing Enclosures for Specialist Electronic Equipment Anti Static Component Trays. Film and Media Industry Manufacture of Costumes and Sets Animation Models and Mock Ups for Computer Simulation Furniture Manufacturing Industry Chair and Seat Backs Cutlery tray inserts Kitchen Unit Panels and Storage Modules Hospitals and Medical Applications Radiotherapy Masks for Treatment of Cancer Patients Pressure Masks for Burn Victims Prosthesis Parts Dental Castings Parts for Wheelchairs and Medical Devices for the Disabled
Packaging and related Industries Point of Purchase Trays and Plates Cosmetic Cases and Packages Electronics and Cassette Holders Blister Pack Products, Skin Pack Products Food Trays, Cups and Fast Food Containers Plastic Sheet Extrusion Testing and Sampling of Extruded Sheet Sanitary Industry Bathroom Fittings Bathtubs, Jacuzzis and WhirlpoolsBath Shower Surrounds, Shower Trays and Retrot Shower Components Signmaking Industry Vacuum Formed Exterior Signs Point of Sale Displays
Machinery Manufacturers Fabricating machine guards and electrical enclosures.
Souvenir Industry Making parts for and moulds to cast craft souvenir’s.
Model Car and Aircraft Industry Production of bodies fuselages and other parts for models.
Theatre Manufacture of Props, Sets and Costumes
Museums Variety of applications within Science and Natural History Museums
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Section 3
The Vacuum Forming Process In its simplest form the process consists essentially of inserting a thermoplastic sheet in a cold state into the forming clamp area, heating it to the desired temperature either with just a surface heater or with twin heaters and then raising a mould from below. The trapped air is evacuated with the assistance of a vacuum system and once cooled a reverse air supply is activated to release the plastic part from the mould. The process is shown in diagram form on g. 3.1 page 13. In its advanced stage pneumatic and hydraulic systems complimented with sophisticated heat and process controllers allow high speed and accurate vacuum forming for those heavy duty and high end volume applications.
Despite the above disadvantages vacuum forming offers several processing advantages over such others as blow, rotational and injection moulding. Fairly low forming pressures are needed therefore enabling comparatively low cost tooling to be utilised and relatively large size mouldings to be economically fabricated which would be otherwise cost prohibitive with other processes. Since the moulds witness relatively low forces, moulds can be made of relatively inexpensive materials and mould fabrication time reasonably short. This results in comparatively short lead times. It provides the perfect solution for prototype and low quantity requirements of large parts as well as medium size runs utilising multiple moulds. (Moulds are discussed in greater detail in section 5). The typical process steps can be identied as follows: clamping, heating with sheet level activated , pre-stretch, forming with plug assist, cooling with air and spray mist, release and trimming They are examined more closely under the following sub headings.
The thermoforming industry has developed despite two fundamental shortcomings. Many other thermoforming processes use a resin base in powder or pellet form. Vacuum forming begins further down the line with an extruded plastic sheet which incurs an additional process and therefore an extra cost to reach this stage. In addition, there is generally an area of material which is cut away from the formed part which unless reground and recycled has to be considered as waste and accounted for in any costings made. However these problems have been invariably resolved by strict control of sheet quality and by clever mould design to minimise the amount of waste material. Throughout this manual you will nd useful hints and techniques to assist in maximising the potential from this process.
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Section 3/The Vacuum Forming Process Clamping The clamp frame needs to be sufciently powerful enough to handle the thickest material likely to be formed on the machine – up to 6mm with our single heater models and up to 10mm with the twin heater machines. If an automated process is used the operation of the moving parts must be guarded and interlocked to avoid accidental damage. In addition a safety guard ( in the form of a fabricated guard or light curtain) must be provided to protect the machine operator at all times. Heating Heaters are generally infra-red elements mounted within an aluminium reector plate. In order to obtain the best vacuum forming results, using any material, it is essential that the sheet is heated uniformly over its entire surface area and throughout its thickness. In order to achieve this it is necessary to have a series of zones which are controlled by energy regulators. Ceramics do have some disadvantage in that their high thermal mass makes them slow to warm up (approx 15 minutes) and slow in their response time when adjustments are made.
controlled precisely using thyristor modules. Full feedback is available to allow zones to be banked up or down by percentage amounts. Twin heaters are also recommended when forming thicker materials as they assist in providing more uniform heat penetration and faster cycle times. Twin quartz heaters as used in the Formech FDH model are advisable when forming high temperature materials with critical forming temperatures. By close control of areas of heat intensity, heat losses around the edges caused by convection air currents and absorption from clamp areas can be fully compensated for and consistent results achieved on a continuous basis. Cost savings can also be considerable if quartz heaters are specied as there is an adjustable percentage power drop when the heaters are in the rear position during the forming process.
More sophisticated quartz heaters are available which have less thermal mass enabling more rapid response time. Pyrometers enable accurate heat temperature control by sensing the melting temperature of the sheet and interacting with the operating process control. A cooling jacket is required for the pyrometer. Precise temperature readout is also available with a computer controlled system working in unison with the pyrometer(s). The megapoint system was devised by Formech for the accurate heating of large areas, using a standard PC and a minimum of exterior electronics. The heating control system is an extension of the process controller, allowing rapid visual interpretation of the heater zoning. Temperatures are
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Section 3/The Vacuum Forming Process Sheet Level A photo-electric beam is incorporated in the machine to scan between the bottom heater and the sheet of plastic. If the sheet of plastic sags down and breaks the beam then a small amount of air is injected into the bottom chamber, thus lifting the sheet to stop it from sagging. Pre-stretch ( Bubble) Once the plastic has reached it’s forming temperature or ‘plastic’ state it can be pre- stretched to ensure even wall thickness when the vacuum is applied. Pre-stretch is an invaluable feature when forming deep draw parts with minimum draft angles and high mould surface detail. The method of controlling the bubble height should be such that consistent results are obtainable. Vacuum, air pressure and optional aids such as a plug assist are then used to assist in moulding the heated, stretched plastic. Vacuum Once the material is suitably pre-stretched a vacuum can be applied to assist in forming the sheet. A dry vane vacuum pump is used to draw the air trapped between the sheet and the mould. The vacuum pump should be capable of maintaining a differential pressure of approx 27” mercury. With larger machines a vacuum reservoir is used in conjunction with a high volume capacity vacuum pump. This enables a two stage vacuum to be applied ensuring rapid moulding of the heated sheet ( before the sheet temperature drops below its ideal forming temperature).
produced without webbing and with even thickness distribution. The idea behind the process is to feed as much material into the cavity prior to the vacuum being applied in order to avoid thinning in that area. Plug moulds are generally made from wood or metal and a smooth surface allows the sheet to slide whilst stretching into the mould. A felt or leather lining ensures that the risk of premature chilling on contact is greatly reduced. Resin plugs provide a good alternative as being good insulators they do not affect the temperature of the sheet. Plug assist is also an essential feature when forming multiple impression male moulds as they can be placed very close together without the fear of the material webbing between the formed parts. Cooling and Release Once formed the plastic must be allowed to cool before being released. If released too soon then deformation of the moulding will result in a reject part. To speed up the cooling cycle high speed fans are tted and activated once the part is formed. A spray mist option is also available whereby nozzles are attached to the fans and a ne mist of chilled water is directed onto the sheet. This, in conjunction with the fans can speed up the cooling cycle by up to 30%. Mould temperature control units are also available which regulate the temperature within the mould ensuring accurate and consistent cooling times when cooling crystalline and crystallising polymers such as PP, HDPE and PET.
Plug Assist Plug-assist forming is the term used to describe the use of a male plug tool, mounted on a pneumatic or hydraulic cylinder situated over the forming area of the machine, to force the material into a female cavity within the moulding area. It enables complicated and deep-draw moulds to be
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Section 3/The Vacuum Forming Process Once cooled sufciently the sheet can be released by a reverse pressure activated through the vacuum system. The part is then stripped from the mould and transferred to the trimming station. See the relevant section for further details on trimming and nishing. The following diagrams display the basic principles of vacuum forming. Fig. 3.1
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Section 4
Plastic materials and their characteristics Plastics comprise a of a wide range of materials but fundamentally fall into two groups – thermoset and thermoplastic, the latter being a material which, due to the molecular structure, has the property of softening repeatedly when heated and hardening once cooled. Thermoplastics also have what is known as a ‘memory’ enabling a formed part to revert to its original state when reheated. It is the thermoplastic type that is used specically for thermoforming and therefore we will concentrate on this category in this section. Further information on the complete range of polymers can be obtained from the contacts listed at the end of this manual. Polymers are made up of molecules which in turn are made up of atoms. These atoms have many different combinations which all have different properties and contain a wide range of additives to give each material its own characteristics. There is constant research being carried out to develop new materials suited to an ever increasing range of applications. Later in this section we have provided a breakdown of the more common materials used for thermoforming, their characteristics and the applications to which they are most suited.
Thermoplastics are split into two different groups – amorphous and crystaline. Crystalline thermoplastics contain an ordered manner of molecules and amorphous contain a random arrangement. Generally speaking amorphous materials, e.g. Polystyrene and ABS are easier to vacuum form as they do not have such a critical forming temperature. When heat is applied amorphous materials becomes soft and pliable – when it reaches this state it is known as its Glass Transition Temperature (Tg). If heated to a higher temperature it reaches a Viscous state (Tv). The changes occur over a range of temperatures and enable the operator to have a fairly wide forming range. Semi-crystaline and crystaline materials, e.g. Polyethylene and Polypropylene have a far more critical forming temperature as they go rapidly from the Tg state to Tv a change known as the Melt Transition Temperature (Tm). When using crystalline materials is imperative that accurate temperature control is used to monitor the heating process. In summary, the forming temperature bands for amorphous materials is much wider and as a result are easier to process in comparison to their semi-crystalline counterparts. In other words they have a much better melt strength and will not sag as much as the melt transition temperature is reached. The accompanying table 4.1 lists some more commonly used amorphous and semi crystalline materials and provides a guideline as to their temperature characteristics.
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Section 4/Plastic materials and their characteristics Table 4.1
Different thermoplastics have different characteristics and are better suited to specic applications. Ideally the material should be easy to form with a low forming temperature, good ow characteristics and thermal strength, high impact strength and low shrinkage on cooling. To improve thermal stability in certain materials like, for example, PVC, stabilisers are added to help prevent degradation when heated. Certain materials are known as Hygroscopic – namely that they absorb moisture which if not predried prior to forming will result in moisture blisters which will pit the surface of the sheet resulting in a reject part. It is a common misconception that the blisters are as a result of too much heat. This in turn can lead to incorrect heating cycles being entered which in turn cause problems with denition on the nished part. ( To avoid the pitting the operator is forced into forming the part before the plastic has reached its forming temperature).
In the following pages we will look at the more commonly used plastics and list their properties, features and some of the more popular applications for which they are used. ( a full schedule of applications for thermoformed products is listed in section 2) For more detailed information and specication we recommend you contact the various suppliers who would be happy to assist with any enquiries. In table 4.2 you will nd a selection of more commonly used materials and the heating times required using a single heater Formech machine equipped with Infra-red ceramics ( if twin heaters are used then the heat cycle can be reduced by up to 30%). These are given as a guideline only as many different grades of materials exist and other factors which affect timescales. We would recommend you contact the plastic manufacturer to obtain more accurate gures prior to commencing production.
To overcome this problem it is therefore necessary for hygroscopic materials to be pre – dried in an oven before forming. The drying temperature and length of drying time depends on the material and the thickness. It is advisable to contact the suppliers who will advise exactly what drying temperature and time is required for their materials. E.g.: Polycarbonate with a thickness of 3mm would require 4 hours at a drying temperature of 80-120°.
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Section 4/Plastic materials and their characteristics Table 4.2
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Section 4/Plastic materials and their characteristics
Acrylonitrile Butadiene Styrene– (ABS) Properties Hard, rigid amorphous thermoplastic with good impact strength and weather resistance. It contains a rubber content which gives it an improved impact resistance. Available with different textures and nishes in a range of thickness. Needs drying. Available in Fire Retardant and UV stabilised grades. Formability Good – forms to a high denition. Hygroscopic Yes – pre drying required at approx 80° (1 hour per mm) Strength Good – High Impact Shrinkage Rates 0.3 – 0.8% Availability From stock Solvent/ Filler Methyl Ethyl Ketene (MEK), Toluene and Dichloromethane Solvent will make ller paste.
Finishing/ Machining Machines well with Circular Saws, Routers and Band saws– takes all sprays. Can be Guillotined and Roller cut. Clear Not Available Colours Black / White / Grey and limited colours. Applications Luggage, Caravan Parts, Vehicular Parts, Sanitary Parts, Electrical Enclosures. Price Medium Stockist Stephen Webster, Amari Plastics, Larger quantities – Doeex
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Section 4/Plastic materials and their characteristics
Acrylic - PMMA – (Perspex, Oroglas, Plexiglas) Properties A high quality hard amorphous plastic with good clarity that can be worked after forming. NOTE: Only extruded sheet is suitable for vacuum forming effectively. Cast Acrylic will not respond well as it displays a very small usable plastic zone. As a result it will only produce general contours with large drape radii. Needs drying. Often replaced by PETG – see separate heading. Formability Tends to be brittle and is temperature sensitive. Hygroscopic Yes – Consult supplier for drying times. Strength Medium to High strength Shrinkage Rate 0.3 – 0.8% Availability Ex stock – 2 weeks Solvent/ Filler Tensol, Solvent and gap ller.
Finishing/ Machining Prone to Shatter. Takes cellulose and enamel spray. Good for hand working. Clear Yes. Colours Solid colours Applications Signs, Roof Lights and Domes, Baths and Sanitary Ware, Light Diffusers Price Expensive Stockist Amari, Visijar Tuckers, Multiplastics
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Section 4/Plastic materials and their characteristics
Co-Polyester – (PETG / VIVAK) Properties An easy forming amorphous thermoplastic. FDA approved for food applications. Optically very good with excellent fabricating performance. Thermoforms with ease utilising low temperatures and fast cycle times. Can be sterilised and is resilient to a wide range of acid oils and alcohols. Not recommended for use with highly alkaline solutions. Formability Very Good – forms to a high denition. Forming range 80 - 120°C Hygroscopic Not normally required. If sheet is exposed to high humidity conditions for an extended time then pre-drying is required – 8 hours at 60° C.
Finishing/ Machining Can be Guillotined, Saw Cut or Routered. Die Cutting and Punching also possible up to 3mm. Paints and Inks for Polyester can be used for printing on PETG. Clear Yes
Strength Good – High Impact
Colours Limited – Contact Supplier
Availability From stock
Applications Point of Sale and Displays, Medical Applications
Solvent/ Filler Cementing can be done using solvents or commercial glues. Can be Ultrasonically Welded.
Price Expensive – ( competitive with other clear materials e.g. PC/ PMMA Stockist ABG, Axxis ViVak
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Section 4/Plastic materials and their characteristics
Polystyrene– Polyphenylethene (H.I.P.S / BEXTRENE) Properties One of the most widely used materials An easy forming amorphous thermoplastic. Thermoforms with ease utilising low temperatures and fast cycle times. Available with different textures and patterns. No pre drying required. Poor UV resistance –not suitable for outdoor applications. Formability Very Good – forms to a high denition. Forming range circa 150°C
Finishing/ Machining Needs special etch primer before spraying. Good machining with all methods.
Hygroscopic No
Clear Yes – Styrolux ( Clarity not to quality of PETG/ PC/ PMMA
Strength Medium to Good impact strength Shrinkage Rate 0.3 – 0.5% Availability From stock Solvent/ Filler Dichoromethane, Toluene. Filler can be made from dissolved plastic in solvent.
Colours All colours and also available in a Flocked nish ideal for presentation trays and inserts. Applications Low cost and disposable items, toys and models, packaging and presentation, displays. Price Low - Medium Stockist Stephen Webster, Larger quantities – Doeex.
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Section 4/Plastic materials and their characteristics
Polycarbonate – (P.C. / LEXAN/ MAKROLON) Properties Hard, rigid clear amorphous material with high impact resistance and good re rating. Self extinguishing. Requires high forming temperatures. Needs drying. Excellent clarity. Similar properties to Acrylic.
Formability Good Hygroscopic Yes – Drying temperature 90° C. 1mm – 1 hr. 3mm – 4hrs. 4mm – 10hrs. Strength Very good impact strength Shrinkage Rate 0.6 – 0.8%
Finishing/ Machining Good for screen printing. Good machine qualities. Can be ultrasonically welded, drilled and tapped. Takes spray. Clear Yes Colours Translucent and solid colours. Opal and diffuser patterns. Available in a variety of embossed textures. Applications Light diffusers, Signs, Machine Guards, Aircraft trim, Skylights, Riot Shields, Guards and Visors
Availability From stock Solvent/ Filler Between PC components Dichloromethane or MEK solvent. Care must be taken with solvents as PC is a stress sensitive material and can be adversely affected by the solvents at its weak points. Most proprietary adhesives can be used to join PC with metal, glass and wood.
Price Expensive Stockist Amari , Comco
SEE PETG AS AN EXCELLENT ALTERNATIVE TO PC
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Section 4/Plastic materials and their characteristics
Polypropylene – (PP) Properties PP is a semi-crystalline thermoplastic which has difcult form characteristics with sheet sag inevitable. Chemically inert and very exible with minimum moisture absorption make it suitable for a wide range of applications. High forming temperature but no drying required. Many grades of PP are available containing llers and additives. Co polymer as opposed to homo-polymer PP is recommended for vacuum forming, as the copolymerisation process helps reduce stiffness and broaden the melt and glass transition temperatures increasing thermoforming ability. Formability Difcult – Translucent material goes clear when in its plastic state – occurs within temperature band of approx 10°C and provides excellent indicator to forming temperature. Good temperature control required in conjunction with a sheet level facility. Hygroscopic No Strength Very good impact strength Shrinkage Rate 1.5 – 2.2% Availability From stock
Finishing/ Machining Does not take spray Clear Translucent – Colours Black / white and colours available Applications Luggage, Food Containers, Toys, Enclosures, Medical Applications, Chemical Tanks. Price Inexpensive Stockist Doeex
Solvent/ Filler No solvent
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Section 4/Plastic materials and their characteristics
Polyethelene – (PE, HDPE, LDPE, PE FOAM) Properties PE is a semi-crystalline thermoplastic with similar forming properties to PP. Good heat control with sheet level required for successful forming. High shrinkage rates but good chemical resistance and strength. Available also as a cross linked closed cell foam (PLASTAZOTE) - ideal for packaging and liners. Formability PE – Difcult PE FOAM – Good but form at lower temperatures to prevent surface scorching. Hygroscopic No Strength Very good impact strength Shrinkage Rate LDPE - 1.6 – 3.0% HDPE - 3.0 – 3.5% Availability From stock Solvent/ Filler No solvents
Clear Translucent – Goes clear when in its plastic state – occurs within temperature band of approx 10°C and provides excellent indicator to forming temperature. Colours Black / white and colours available Applications Caravan Parts, Vehicular Parts, Enclosures and Housings. Price Inexpensive Stockist PE - Amari, Simona PE FOAM – Polyformes CONDUCTIVE PE – Dentec
Finishing/ Machining Does not take spray. Takes some specialist inks.
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Section 4/Plastic materials and their characteristics
Polyvinylchloride – (PVC) Properties Strong, tough thermoplastic with good transparency in thinner gauges. Good chemical and re retardant properties. Highly resistant to solvents. Thicker materials are rigid with good impact strength ideally suited to outdoor industrial applications. Formability Forms well but with a tendency to web.
Finishing/ Machining Does not take spray. Takes some specialist inks.
Hygroscopic No
Clear Yes – Different web widths available with thickness from 150 microns – 750microns.
Strength Good Shrinkage Rate N / A - Contact Supplier Availability From stock – Sheet or Reel Solvent/ Filler Toluene may be used – no others solvents suitable. Hot air weld or glue.
Colours Black / white and colours available Applications Packaging, Machine Guards and Car Trim. Price Inexpensive Stockist Smaller quantities – Stephen Webster
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Section 5
Mould and Mould Design
Mould Materials Selection of the best suited mould material depends largely on the severity and length of service required. If only a few parts are required using fairly low temperature plastics then wood or plaster could be used. However, if the quantity requirements run into the thousands and material temperatures are higher then ideally an aluminium based resin or aluminium mould would be recommended.
The thermoforming mould can be as simple as a wooden block or as sophisticated as an injection mould with all the ancillary elements to enable in mould trimming. They are one of the most important parts of the thermoforming cycle. One of the main advantages of vacuum forming is that the pressures used are signicantly less compared to, for example, the injection moulding process. The result is that vacuum formed tools can be produced economically and in a wide range of materials to suit different prototype and production requirements. In this manual we concentrate on moulds ideally suited to the vacuum forming process. The prime function of a mould is to enable the machine operator to produce the necessary quantity of duplicate parts before degradation.
Once a prototype mould has been fabricated then it is a simple process to cast a resin mould into a forming taken from the original tool. See the heading Resin moulds for further details.
A wide range of materials can be used but it is important to determine the correct mould material and type most suitable for a particular application. In this section we look rstly at the different types of mould material available. We then look more closely at different types of moulds, mould design and techniques and provide some useful tips and hints to assist the ‘in house’ production of moulds.
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Section 5/Mould and Mould Design
1) Modelling Clay, Plaster Modelling clay is widely used for educational and model making purposes. It enables the user to quickly shape a low cost prototype which can then be cured in an oven overnight. Suitable only for a few formings as the heat and pressures applied cause it to deteriorate rapidly. Plaster is a good material for making inexpensive prototype moulds. However it is essential that the plaster is allowed to dry in a warm environment for up to three days. The reasons are twofold; 1) Moisture can be drawn into the vacuum system causing internal damage to the machine and pump. 2) The time is required for the plaster to develop nal properties and stabilise the water content. When using plaster moulds it is also essential to have a lter tted to ensure no powder or particles are drawn into the vacuum system. The surface of a plaster mould is sensitive to heat build up and therefore tend to crack and break up after about 50 cycles. It is not normally necessary to vent plaster moulds as the surface is porous.
2) Wood Wooden moulds are cheap and easy to fabricate, and have a longer life than plaster moulds – in many cases being used on a production basis for in excess of 500 formings. Hardwoods are recommended, notably ‘jelutong’ and ‘obeche’ which both have a close and even grain which makes them easier to work and less prone to cracking and splitting during the forming process. Conventional woodworking techniques are used to fabricate the moulds. It is important to ensure the wood is kiln dried before working to ensure there is no warping or cracking during fabrication. As with any wood due to expansion and contraction during the forming process deterioration is inevitable but can be reduced by sealing with an enamel or varnish. This will enable countless mouldings to be produced with minimum renishing required. Because of the cost implications there are many cases where for particularly large applications such as signs, displays and whirlpools which require thicker materials wooden moulds are used on a production basis. Grease, parafn and vaseline and silicone release sprays can all used as a release agent.
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Section 5/Mould and Mould Design
3) Cast Epoxy Resins There are numerous resins available which are relatively cheap and easy to work. Moulds made from this material are durable and produce a forming with good surface nish. Some synthetic resins are sensitive to surface heat build up but this can be alleviated by incorporating aluminium powders to increase the heat stability and also the longevity. They are normally supplied as a two part mix; the resin itself and a hardener.
The plastic moulding should be at least 2mm in thickness, mounted into a wooden frame lled with sand for support to avoid distortion. It is then necessary to mix the resin and hardener according to instructions and then allow time for curing. With larger moulds and to save on resin costs and reduce mould weight it is normally advisable to ll the mould with wooden blocks or foam around which the resin is poured.
Once an original pattern has been produced either in wood or other material it is possible to use a forming taken off this as a mould in which to cast the resin. You can see from the accompanying pictures the process in its different stages. The pictures are supplied courtesy of Ciba Geigy who produce a compatible resin XD4500 for vacuum forming applications. Alchemie also supply a range of resin tooling systems suitable for vacuum forming moulds. (see the suppliers section for company details.)
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Section 5/Mould and Mould Design
Fig. 5.1
MIX Select the pack size which is appropriate to the casting being made. The pack volumes are given below as a guide : 5kg pack - 3lt 2kg pack - 1.2lt
MODEL Models may be made from a variety of patternmaking materials, e.g. wood, plastics, and metal etc. Porous materials should be sealed before use, for instance with a polyurethane varnish. Care taken at the model making stage will be reected in the quality of the nished tool. The next stage depends on whether a male or female production tool is required. To produce a female tool, apply a wax release agent or a suitable alternative to the model in preparation for the casting operation. A male tool can be made by taking a vacuum forming from the model. The forming should be backed with a material such as plaster of paris for rigidity and then be released as above for casting.
Pour all of the hardener into the resin container and add accelerator according to the thickness of the cross section to be cast. Mixing instructions are enclosed with each pack and these dene accelerator additions. Stir thoroughly taking care to mix in resin from the sides and bottom of the container. After stirring allow the mix to stand for 3-5 minutes to enable air bubbles to rise and break. Alternatively, if the equipment is available de-aerate in a vacuum chamber. The mix should be cast well within its pot-life which is 40 min at 21 °C. As with all resin systems the mix is exothermic in bulk and this effect is increased by higher temperatures and by the use of XD 4500 accelerator.
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Section 5/Mould and Mould Design
Fig. 5.2
FINISHING CURING. Allow the resin casting to harden at room temperature and then postcure as follows: 16 hours at room temperature - to demould 4 hours at 40 °C 3 hours at 60 °C 2 hours at 80 °C
CAST Pour the mixed resin system slowly and in a steady stream into the lowest point of the mould until the required tool thickness is reached. For mould surfaces with ne detail, rst carefully brush a thin coat of mix over the surface and then proceed as above. Castings of greater thickness than the normal maximum of 3 in (75mm) can be made by pouring subsequent layers of mix onto the back of the preceding one, once it has gelled but not fully hardened. Wood or polystyrene blocks can be suspended in the mould cavity to reduce cross sectional thickness. This serves to reduce the risk of excessive exotherm and to economise on resin usage.
The above times are minimums. In the case of tools with a cross section thickness of less than 1 in (25mm) the room temperature stage must employ a temperature of at least 15 °C to provide an adequate initial cure for demoulding and further processing, e.g. machining etc. When time is at a premium tools may be post-cured by putting them into service. Care is essential to avoid overheating during the early stages of tool life. DRILLING AND MACHINING. Evacuation holes may be drilled by any one of the engineering techniques available. Araldite Vacuum Forming Tooling System XD 4500 is formulated to be easy and clean to machine. It contains no hard llers and produces swarf rather than dust.
25
Section 5/Mould and Mould Design
Fig. 5.3 XD 4500 in use on a small scale vacuum former
Araldite Vacuum Forming Tooling System XD 4500 is designed to meet a particular need of the toolmaker for small to medium size cast resin tooling. The Araldite Tooling resin range also includes resin systems suitable for the production of larger vacuum forming tools. These are produced by employing gel coats, laminating systems in combination with fabrics, and sometimes aluminium pellets, to provide a variety of materials and techniques for many applications
26
Section 5/Mould and Mould Design 4) Aluminium Aluminium is frequently the material chosen for production tooling due to its good surface hardness, heat conductive properties and low wear. It is lightweight and has an excellent strength to weight ratio. It can be machined from blocks or cast from patterns and due to its thermal properties heat from the formed plastic sheet is quickly and efciently dissipated. A wide range of surface nishes are possible but generally speaking a sand blasted surface is ideal in that it prevents air being trapped between the mould and heated sheet. Aluminium moulds have a virtually unlimited lifespan. 5) Metaphor – Porous Aluminium Metaphor – F100 Al is a micro porous air permeable aluminium. It is an exciting new breakthrough in material science, where a material has been custom designed specically to enhance the performance of tools for vacuum forming. Although expensive it has the following advantages over aluminium; • It permits more accurately nished thermoformings because the micro vents are micro close together, allowing the material to be rmly held in place over the entire tool during tooling. • It machines faster than aluminium and does not require venting as it is naturally porous. • More intricate moulds with steeper draughts are possible.
Fig. 5.4 A metaphor mould demonstrated on one of the Formech range of vacuum forming machines
It is available in slab form and can be obtained through the UK supplier, Alchemie. See the suppliers section for company details.
27
Section 5/Mould and Mould Design Male and Female Moulds There are basically two kinds of moulds: male (positive) and female (negative). See g 5.5 below.
Fig. 5.5
Male Mould
Female Mould
Several factors will affect the decision as to which is more suited and below we provide a few useful pointers. The top surface of a moulding (the part not in contact with the mould) is invariably the better nished surface, since it cannot pick up any marks such as dust particles from the tool itself. This factor alone may dictate whether a male or female mould is required. Often a male tool is much easier to make and more suitable for a single deep-draw object. On the other hand, a compartment tray with divisions, would typically be of female construction. Fig 5.6 shows a male and female forming and the effects of thinning to the plastic sheet.
Fig. 5.6 A greater degree of denition is achieved on the side of the plastic in contact with the mould. The choice of a male or female should be considered so that the side requiring the highest denition is the one in contact with the mould especially thicker plastics.
28
Section 5/Mould and Mould Design In general, a mould cavity which is deeper than its diameter will give unacceptable thinning at the bottom corners. Negative moulds will produce a forming progressively thinner towards the bottom, because, directly vacuum is applied, the material will cling to the sides of the mould and will tend to stretch like a piece of elastic. To produce a more uniform thickness a plug should be used to stretch the material mechanically before vacuum is applied. On a positive mould and especially if the Pre-Stretch (bubble) option is used this mechanical stretching is done automatically. It may be worth discussing your mould requirements with the material manufacturer if in any doubt. Baseboards and Mounting Generally speaking moulds should be mounted onto baseboards prior to forming to assist release. However, from time to time and often when a quick prototype is required moulds are placed directly onto the mould table and formed over. The main setback with this method is that when it comes to releasing the cooled part from the mould it often, due to shrinkage, sticks to the mould. It is then necessary to remove the part with the mould inside and physically split the two or trim the part whilst attached to the mould. e.g. In the case of radiotherapy mask moulds which have many undercuts and are placed directly on the table, the part and mould are removed together and trimmed out with an air powered hand operated slitting saw.
Depending on which machine you have will determine which size the baseboard should be. We advise that if, for example, you are utilising the full forming area (620mm x 620mm) with the model 660 machine then the baseboard should be made to t tightly into this aperture. To improve further the release the baseboard can be mounted directly to the table. In order to do this the table needs rst to be drilled and tapped in the four corners which act as the location points for the baseboard. When mounting moulds to the baseboard it is necessary to ensure there is some clearance for airow between the mould base and the board. This can be done by either using a thin gauze or by incorporating channels.
In most cases it is recommended to use a baseboard. The baseboard can be made from hardboard or steel plate. It’s primary purpose is to locate and hold down the mould when using the reverse blow facility. We recommend that a thickness of between 3 – 5mm is used to ensure it sits ush with the top of the forming area seal on the machine.
29
Section 5/Mould and Mould Design Draught Angles / Tapers Most moulds are made with a base to sit at on the forming table and must be provided with a draught or taper to facilitate removal, (g 5.7.) The degree of taper will depend on various factors, such as the surface quality of the tool, the depth of near vertical faces, type of material used and if the option of pre-stretch is being utilised. In some instances, an internal recess may be made with zero draft angle, since the shrinkage will actually pull the sheet away from the mould. However the minimum typical taper we would recommend to ensure good quality forming and moulding release would be circa 5°. It therefore goes without saying that the greater the taper, the more even the thickness of sheet and the easier it will be to release. In summary, if using female moulds we recommend a minimum taper of 3° and 5° for male moulds.
Fig. 5.7
Venting An important feature of mould design is the requirement for suitably positioned vent holes to facilitate the evacuation of air trapped between the plastic sheet and the mould. Ideally located in parts where the sheet last makes contact - notably edges, cavities and internal corners. All these areas need to be vented to ensure good denition and rapid air evacuation. ( g 5.8 and 5.9)
Fig. 5.8
30
Section 5/Mould and Mould Design
Fig. 5.9 Typical Vent Whole Position
Depending on plastic used and mould design determines the number of vent holes required. Ideally they should be as few as possible and small enough to prevent them witnesses on the nished parts outer surface. However if too few vent holes are provided or if the vent area is too small, the rate of draw-down will be controlled by the rate of air owing from the bubble. If this is too slow then the plastic may cool before the required denition has been achieved. The diameter of vents at the surface should be less than half the material thickness at the mould surface or between ½ and 1mm. They can be far larger below the surface and one solution is to drill the smaller hole from the surface using a high speed hand powered drill or pillar drill. The mould can then be inverted and a larger hole drilled from the underneath. As an alternative to a drill it is possible to use spring steel otherwise known as piano wire. This material when attened and sharpened is ideal for venting difcult angles and for creating evacuation holes in deeper moulds when drill lengths restrict the venting depth.
Shrinkage and Mould Release On cooling and hardening, a molding will tend to shrink on to a male mould. Different thermoplastics have differing shrinkage rates depending on the grade and thickness. Crystalline and semi crystalline materials tend to shrink more than amorphous normally due to the higher forming temperatures required. The shrinkage rates of some of the more widely used plastics are listed in the plastics section under the specied materials, however, we do recommend you contact the supplier for more accurate gures as different grades of material may have different shrinkage rates. The shrinkage rate of the materials will also affect the mould design in that these differences need to be taken into consideration during the design phase especially if tolerances are critical. Difculties in stripping the molding from the tool will depend to a large extent on mould design. If generous tapers, no undercuts, good surface nish exist then removal should be fairly straight forward. In order to assist removal there are a number of oil and silicone based release sprays which when applied to the mould prior to forming facilitate easy release.
31
Section 5/Mould and Mould Design It is also possible to use a compressed air line to blow air between the molding and the tool. The most effective way to ensure that the moldings are released on a repeated basis is to ensure that the moulds are mounted on baseboards which in turn can be attached to the mould rise and fall table on the machine. This ensures that only the plastic part is ejected when release is activated. (please cross reference with the sub heading Baseboards and Mounting)
Undercuts, Split and Multi Impression Moulds A number of other features can be incorporated into mould design. Although technically not possible as once formed it is impossible to release, undercuts can be incorporated into a mould design with the use of split tooling. With the use of a removable side entrant tool it is possible to achieve undercuts in forming. Tooling costs are higher in most cases. If the mould has an undercut at one end but an equal angle at the other end then the nished part can also be released. (g 5.10)
Not possible
Possible As direction of removal has a greater angle than the undercut
Fig. 5.10
Multi impression moulds are used when production requirements justify the added expense. It is normally the case that with higher levels of production the maximum forming area of the machine is utilized by making multi impression moulds.
Fig. 5.11
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Section 5/Mould and Mould Design Mould Cooling For large production runs we recommend a water cooled mould. The temperature can be controlled by a chiller unit which is connected directly to the mould. Channels are incorporated in the moulds during manufacture to accommodate this facility. This helps maintain a constant mould temperature ensure consistent results combined with optimum cycle times. It is also possible to mount the mould onto a cooling bolster which contains channels for circulating cooling uid. Costs for these moulds are considerably higher than conventional moulds, however it is normally a justiable expense due to the production levels required.
Plug Assist Design The purpose of a plug feature is two fold. It is used to prevent webbing in the forming of multiple male moulds which are close together and to help achieve good wall thickness when forming into deep cavities. Under normal conditions plastic will start to thin radically once it exceeds in depth more than 75% of the cross section (g 5.12).
Fig. 5.12
* Depth of cavity can be 75% of the width of the opening on the surface. Excess thinning will occur beyond this depth.
Fig. 5.12
The plug is used to push the heated material into a female mould prior or in conjunction with the vacuum being applied. It is used whenever large draw ratios are required. Draw Ratio = depth of the aperture divided by the length of the shortest cross section. For example a refrigerator liner has a large draw ratio in that it is a deep molding with small cross section.
However on smaller machines with manual operation the plug is often operator handled on a manual basis. The majority of plug moulds are simple in design and made from hardwood. A felt or ocked surface is often added to ensure the plug glides into the aperture without tearing or marking the plastic too much.
In most cases the plug assist facility is a feature suspended above the forming area and activated by pneumatic or hydraulic systems. (g 5.13)
33
Section 5/Mould and Mould Design
Fig. 5.13
34
Section 5/Mould and Mould Design Webbing/ Chill Marks/ Thinning Webbing Another factor which affects mould design is webbing. This occurs when the material shrinks back on itself. The cause of this can be any of the following; • When the mould is too high in relation to its base area. • By sharp vertical corners with minimum draught angles • By deep multiple male moulds in close proximity to each other.(Fig 5.14) • Excess of sheet material when using small moulds.
Fig. 5.14
A female mould Fig 5.15 can often overcome the webbing caused by multiple male moulds.
Fig. 5.15 Webbing can be voided by restricting the amount of sheet around the mould. This can be in the form of reducing windows (g 5.16) or with the use of a plug. It can be minimized by a slower application of vacuum or by using thicker sheet. It may be necessary to modify the mould design if all else fails to solve the problem.
Fig. 5.16
35
Section 5/Mould and Mould Design Chill Marks When raising a deep draw mould into a heated sheet, the point which rst makes contact cools, reducing its ow characteristics and producing a uneven ow of material at the top which thins down the sidewalls as the vacuum is applied. (g 5.17) This can be eliminated by increasing the draught angles and ensuring the mould is not too cool. The pre-stretch facility (bubble) is also a useful feature in overcoming this problem.
Fig. 5.17
Thinning This is one of the most common problems with mould design and is the result of deep draw ratios and minimum draught angles. See g 5.18 shows thinning caused as a result of deep draw male mould with minimum draft angles. There are numerous solutions which include pre-stretch ( bubble), plug assist , strengthening ribs incorporated into the mould design and increased internal draught angles.
Fig. 5.18
The above three problems are examined further in the trouble shooting section of this manual. Formech offer a pattern making facility with moulds from wood, resin and aluminium. For all your mould requirements please contact us at :
[email protected]
36
Section 6
Finishing and Trimming
Finishing and Trimming With vacuum forming there are secondary processes and operations required before a nished part will be ready for the customer. Once the formed part has cooled and been removed from the machine the excess material is removed, holes, slots and cut-outs are drilled into the part. Other post forming processes include decoration, printing, strengthening, reinforcing and assembly. A variety of different trimming methods are used to trim the product from the sheet. The type of equipment best suited depends largely on the type of cut, size of the part, draw ratio, thickness of material and the production quantity required. They are also factors to consider when determining the investment cost of such equipment. Below are listed some of the more popular methods adopted. Thin gauge parts are normally trimmed on a mechanical trim press – otherwise known as a Rollerpress. Heavy gauge parts can be removed, placed into trim “jigs” or xtures and trimmed with most of the methods listed below.
37
Section 6/Finishing and Trimming Considerations Over time you will usually be able to build up a gut feel for which particular plastic processing method is best suited to your particular product. In order to do a comparison to the other processes that are available, you will need to take note of the following factors: Quantity: The amount of parts you wish to produce are critical in determining not only if vacuum forming is the right process for this particular job, but also the number of tools you need to make and the total forming area. If time is limited, then you may decide to invest in a greater numbers of tools in order to complete the job in less time. Price: Very few customers adopt the approach of “money’s no object”. There is always a good reason why a product has been produced in a particular manner and this is usually cost related. As a general rule once the quality and delivery issues have been established then price usually becomes the deciding factor in which process will be used, although occasionally there are exceptions.
Strength: What’s the application? Will the vacuum forming require additional fabrication in order to give it the rigidity it requires? Will the extra parts be glued , welded or fastened in place? Time: Most processes require CAD these days, either at the design stage or to create the nished tool for production. This all takes time and needs to be added to the total delivery time (including the actual production of the parts) to work out when the parts will be on your doorstep. Example Let’s assume that you intend to produce a vacuum formed machine cover based on the following: Specication: Material: 2mm red high impact polystyrene. Size: 330mm x 180mm x 50mm high. Quantity: 500 off per annum. To be produced in batches of ten at a time.
Finish: What sort of nish do you require? What market is the part going in to? Is the part going to be seen? Is it for indoor or outdoor application? How many post forming operations will be required in order to turn the vacuum forming into the nished part? Accuracy: The more accurate the part the more expensive it becomes to produce. Vacuum forming will not have the consistency of injection moulding, because you are only forming over one face, whereas injection moulding is a totally enclosed die, with toleranced surfaces on all sides.
Machine cover showing the top side and the underside. This part would t comfortably on our 300XQ machine and because of the small quantities and the size of the part we would form the part one up. In this particular case you could only t one tool on the table of the 300XQ. If you wanted
38
Section 6/Finishing and Trimming to produce 500 parts in one go then usually this kind of part would be produced two up on one of our larger machines. The benets of increasing the number of tools used to form the parts is that the job is completed in a shorter time and so you have reduced labour and machine costs and in addition when multiples of tools are used, it usually means that less material is wasted. Post forming operations With most vacuum forming projects using the slightly heavier gauge materials there is always a certain amount of post forming operations to be carried out and in most cases people usually multitask during the machine operation. If you are using an automatic machine you will have all the time between clamping one sheet and the next sheet. A typical automatic cycle will be as follows:
parts stand around untrimmed the more dust they collect and the possibility of damage increases. There is another factor that is often overlooked – boredom. Just spending your life waiting for machines to complete their cycles is tedious. When the operator is active and focussed on the job when multitasking there is a tremendous sense of achievement and the time just ies by. Exactly what is required to complete this machine depends on the equipment you have available. Let’s assume that you do not have access to a ve axis CNC router and that you are going to trim the part with more conventional equipment. (Incidentally a ve axis CNC router would easily trim the part in under three minutes.
1. Place sheet in machine and clamp the material. 2. Heat the material. 3. Pre-stretch the material. 4. Raise the tool up into the material. 5. Apply the vacuum. 6. Start the cooling cycle. 7. Release the formed part from the tool and lower the tool. 8. Open the clamping frame and remove the formed sheet. Repeat action 1 to start the next cycle. If you are forming 2mm material with cooling fans, then you will typically have a 3 minute cycle in which to carry out additional tasks. It would be very wasteful just to let the vacuum formings stack up beside the machine and then do all the trimming at a later date. Try if possible to complete or carry out as many other operations relating to the job as possible during the cycle. The longer
Heated knife/ Scalpel Educational user / Model maker/ R & D work – thin plastics only.
39
Section 6/Finishing and Trimming Fig. 6.3 Vertical Bandsaw
Horizontal Bandsaw (Fig 6.4) This method can be used for both thick and thin thermoplastics formings. The technique has a very wide application, the essential features being a sliding table whose height relative to the saw blade can be adjusted ( or vice versa).
Fig. 6.4
40
Section 6/Finishing and Trimming Rollerpress/ Steel Rule Dies (g 6.5) The machine consists of a pair of adjustable, electrically driven rollers, placed one above the other. The rollers rotate in opposite directions to draw the cutting tool and the material to be cut between them. High quality shape and hole cutting can be achieved with low cost cutting dies. Simple and easy to operate they will form cut virtually all types of exible and semi rigid materials up to a thickness of about 4mm. Cutting widths from 700mm upwards can be accommodated. Please refer to the products section for more information on the range available.
Fig. 6.5
41
Section 6/Finishing and Trimming Guillotine The industrial guillotine is similar to those used to cut paper. Used to cut non brittle and thinner materials. Available in manual and powered versions probably the most well known supplier in the U.K is the company Edwards who have a good range of industrial guillotines. Clicker / Punch Press A moving head which is moved over the trim jig containing the formed part and then activated to press down and cut the part.
Hand Held Air Powered Router This is a versatile and accurate manual method of trimming awkward shapes. The moulded part is located in a ‘trim jig’ with guidelines along which the operator can run the router. A bearing is attached to the router cutter to ensure smooth movement when trimming parts. See g. 6.6-6.9 for examples of products trimmed using this method.
Fig. 6.6
Fig. 6.7
42
Section 6/Finishing and Trimming
Fig. 6.8
Fig. 6.9 De Soutter provide very reliable air tools and have branches nationwide. Bearings and cutters are added to suit. Circular spindle saw As the name suggests this is a circular cutter mounted on a spindle and xed in a pillar drill. The cutting disc is reversed to ensure safety when trimming parts. Cuts vertically from above.
43
Section 5/Mould and Mould Design Circular Cutter mounted from under table. Larger industrial version of the Formech Gerbil and powered by a router motor mounted under a table. Ideal for cutting on a vertical edge. Below we have provided a guideline to the preferred trimming methods for a range of the more commonly used thermoplastics.
Table 6.1
Formech are able to assist with any questions you may have and also provide further details of suitable trimming machines and suppliers. Please also refer to the products section of the web page.
44
Section 7
Trouble Shooting Guide
The purpose of the following section is to help the user in overcoming some problems frequently encountered in thermoforming. Most of the major problems identied in thermoforming can be traced back inadequate process control. Improper sheet temperature at the time of forming is a primary source of problems. Poor vacuum control and insufcient cooling are also typical causes. The table below lists the majority of recurring problems in thermoforming along with the suggested courses of action to correct or eliminate the processing problem.
Table. 7.1 Problem
Cause
Blisters or Bubbles
Overheating/Sheet heated too - Lower the heater temperatures or rapidly reduce the top heater if using twin heating. Excessive moisture - Pre-dry sheet - Pre-heat sheet - Heat from both sides - Keep material wrapped until ready to use Uneven Heating - Check consumption. - Increase zone control. Mould or part too hot - Increase cooling cycle. - Decrease mould temperature. Mould undercuts - Increase release time and pressure Insufficient draft angles - Increase draft. - Convert mould to female. Poor mould surface - Use mould release. Improve mould surface. Top/Bottom surface too hot - Decrease heating cycle time. - Decrease heater temperature. - Check for faulty heat zones if problem in isolated area. Pre-heat material.
Poor Mould Release
Sheet Scorching
Remedy
45
Section 7/Trouble Shooting Guide Problem
Cause
Remedy
Lack of Definition
Material too cold
- Increase heating time. - Increase heater temperature. - Increase heater density and/or wattage. - If localized problem check heater zone and elements. - Ensure mould is at optimum temp. for forming - Adjust vacuum timings. - Check vent holes on mould. - Increase number and or diameter of vacuum holes. - Check mould is not restricting vacuum flow - are vacuum tracks in mould adequate? - Check for leakage in vacuum system - Increase vacuum capacity - Shorten cycle time. - Lower heater temperature. - Check system for leaks - Increase size of vacuum holes - Check for blocked holes - Adjust pre-stretch flow and time - Reduce material size and use reducing windows
Mould too cold Insufficient vacuum
Webbing
Material too hot Insufficient vacuum
Incorrect Pre-Stretch height Excess material Poor mould design
Vacuum speed too fast
- Increase radii improve draw ratios - Use plug/ring assist. - Use assist blocks to pull out webbing. - Increase spacing between moulds. - Switch to female mould. - Regulate to suit. - Use smaller vacuum holes.
46
Section 7/Trouble Shooting Guide Problem
Cause
Whitening
Part removed too early
Chill Marks
Thinning at corners on deep draw parts
Remedy
- Part must be below set temp. before removing - Increase cooling cycle. - Add extra cooling fans - Utilise a spray mist facility - Use water cooled moulds. Uneven part cooling - Increase mould temp. and/or temp. uniformity. Poor mould design - Redesign. Incorporate ribs/tapers Poor sheet distribution - Check for uneven heating - Use plug assist or pre-stretch for deep draw. Excessive mould release pres- - Adjust to suit. sure or timing Stretching stops when sheet - Increase mould temp. makes contact with cold - Increase plug temp. mould/plug Mould design - Use plug to assist mould flow. Insufficient pre-stretch Plug tool too cold Uncontrolled sheet heat
- Increase pressure, flow or time - Wrap tool with felt or similar. - Improve zone control.
Sheet formed too slow
- Use more vacuum - Increase number and size of vent holes - Decrease flow or timing. - Check elements - Check for draughts. - Increase - Use plug assist and/or pre-stretch
Too much pre-stretch Cold spots Too thin gauge Uncontrolled material distribution
47
Section 7/Trouble Shooting Guide Problem
Cause
Remedy
Tearing of sheet whilst forming
Sheet too hot
- Decrease heating cycle time. - Decrease temp. settings. - Pre-heat sheet. - Increase heating cycle time - Increase temp. settings. - Reduce time and flow. - Increase corner radii and draw ratio. - Consult supplier - Increase cooling cycle. - Lower mould temp. - Reduce heating cycle time. - Mount mould to baseboard ensure it is then fixed to table.
Sheet too cold Pre-stretch too large Mould design
Parts stick in mould
Improper material selection Part temp. too high
Mould not fixed on baseboard Insufficient draft Mould undercuts
Poor mould quality
Nipples on formed parts
Sheet too hot
Excessive sag
Vacuum holes too large Sheet too hot Sheet area too large
- Increase taper. - Increase air eject. - Remove part earlier and use cooling jigs. - Use retractable undercut segments in mould. - Polish mould. - Use release agent. - Remove rough edges from wooden moulds. - Reduce heating cycle - Reduce heat temp. Plug holes/re-drill. - Decrease heat cycle time. - Decrease heater temp. - Improve zone control - Ensure auto level feature activated.
48
Section 7/Trouble Shooting Guide Problem
Cause
Remedy
Marks on finished moulding
Dirty mould
- Clean mould with air gun after each cycle. - Decrease spray mist. - Check all hoses, gaskets for leaks. - Smooth/Polish surface. - Inspect handling procedure - Ensure protective film removed immediatly prior to forming. - Check clamp pressure. - Adjust alignment. - Heat frame prior to inserting sheet. - Increase heat cycle. - Consult supplier.
Water droplets Poor mould surface Scratched sheet
Plastic pulls from clamp frame
Insufficient clamp pressure
Material too cold Wrong material selection
49
Section 8
Supplier guide
The following list of suppliers has been divided into four sub sections; 1. Formech Int Ltd – Machines & Spares, Ancillary Equipment. 2. Plastic Stockists, 3. Plastic Extruders (minimum order 500 - 1000 kilos) 4. Resin/ mould material suppliers.
Formech International Ltd Sales and Administration Unit 4, Thrales End Business Park Thrales End Lane Harpenden Hertfordshire AL5 3NS England Tel: + 44 (0)1582 469 797 Fax: + 44 (0)1582 469 646 Email:
[email protected] www.formech.com Spare Parts and Service Unit 4, Thrales End Business Park Thrales End Lane Harpenden Hertfordshire AL5 3NS England Tel: + 44 (0)1582 469 797 Fax: + 44 (0)1582 469 646 Email:
[email protected] www.formech.com Suppliers of: • Vacuum Forming Machines. • Reducing Windows, Spare Parts and Spare Parts Kit. • Rollerpresses, Horizontal Bandsaws and Gerbils • Drying Ovens, Line Bending Equipment. • Tooling – Prototype and Production • Tradeforming facility
50
Section 8/Supplier guide
Plastic Sheet Stockists ABG Ltd Galowhill Road Brackmills Northampton NN4 7EE England Tel: + 44 (0)1604 70 08 80 Fax: + 44 (0)1604 76 61 13 Email:
[email protected] www.abgrp.co.uk Suppliers of: PETG. Axxis Vivak. Clear polyester sheet. Amari Plastic Ltd Branches nationwide. Head Ofce: Holmes House 24 – 30 Baker Street Weybridge Surrey KT13 8AU England Tel: + 44 (0)1932 83 50 00 Fax: + 44 (0)1932 83 50 01 Email:
[email protected] www.amariplastics.com Suppliers of: ABS, PC, PP, PMMA Multiplastics Ltd Paycocke Road Basildon Essex SS14 3EU England Tel: + 44 (0)1268 28 91 81 Fax: + 44 (0)1268 53 04 76 Email:
[email protected] Suppliers of: Acrylic, PC.
Simona Ltd Telford Drive Brookmead Ind. Park Stafford ST16 3ST England Tel: + 44 (0)1785 22 24 44 Fax: + 44 (0)1785 22 20 80 Suppliers of: PE, PP, PETG Stephen Webster Plastics Ltd Brick Knoll Park Ashley Road St Albans Herts AL1 5PL England
Suppliers of: Flocked P.S and PVC (reel form only) Polyformes Ltd Cherry court Way Stanbridge Road Leighton Buzzard Beds LU7 8HU England Tel: + 44 (0)1525 85 24 44 Fax: + 44 (0)1525 85 04 84 Email:
[email protected] www.polyformes.co.uk Suppliers of: Closed cell PE foams.
Tel: + 44 (0)1727 86 31 38 Fax: + 44 (0)1727 84 42 91 Email: sales@stephen-webster. co.uk www.stephen-webster.co.uk Suppliers of: • Holds in stock a large range of Sheet Polystyrene in black, white, colours and textures. • Also supplies ABS, PETG, PVC, Acrylic Capped ABS. • Excellent delivery times and no minimum. • Discounts for quantity. Free delivery over £ 50.00 UKAP Sterling House 21-25 Station Lane Hornchurch Essex RM12 6JL England Tel: + 44 (0)1708 44 77 88 Fax: + 44 (0)1708 44 11 11 Email:
[email protected] www.ukap.co.uk
51
Section 8/Supplier guide
Plastic Extruders
Resin Suppliers
Athlone Extrusions Ltd Grace Road Athlone Co. Westmeath Ireland
Alchemie Limited Warwick Road Kineton Warwick CV35 0HU England
Tel: + 353 (0)902 92 67 9 Fax: + 353 (0)902 94 08 6 Email:
[email protected] Extruders of: P.S. ABS (Fire retardant) Acrylic Capped ABS Doeex PLC Holmethorpe Avenue Redhill Surrey RH1 2NR England
Tel: + 44 (0)1926 64 16 00 Fax: + 44 (0)1926 64 16 98 Email:
[email protected] www.alchemie.com Suppliers of: • Epoxy Resin Tooling Systems for vacuum forming. • Metaphor – Porous Aluminium
Tel: + 44 (0)1737 77 12 21 Fax: + 44 (0)1737 77 24 61 Email: sales.doeex@dial-pipex. com Suppliers of: Polystyrene / Conductive PE and PP Wardle Storeys Brantham Works Brantham Manningtree Essex CO11 1NJ England Tel: + 44 (0)1206 39 24 01 Fax: + 44 (0)1206 39 52 88 Email:
[email protected] Extuders of: HDPE, PP.(Minimum order- 500 Kilos)
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Section 9
Further Information
British Plastic Federation (BPF) 5-6 Bath Place Rivington Street London EC2A 3JE England Tel: + 44 (0)20 74 57 50 00 Fax: + 44 (0)20 74 57 50 45 www.bpf.co.uk
We hope this manual has provided a useful insight into vacuum forming. Whilst we have tried to cover all aspects of the process the following industry bodies and associations are able to provide further assistance and information on the thermoforming process in general. Please see our contacts section for links to the following companies or you can link directly below.
Rubber & Plastics Research Association of G.B. ( RAPRA) Shawbury Shrewsbury Shropshire England Tel: + 44 (0)1939 25 03 83 Fax: + 44 (0)1939 25 11 18 Email:
[email protected] www.rapra.net
British Polymer Training Association(BPTA) Haleseld 7 Telford Shropshire TF7 4NA England Tel: + 44 (0)1952 58 70 20 Fax: + 44 (0)1952 58 20 65 Email:
[email protected] www.bpta.co.uk Institute of Materials 1 Carlton House Terrace London SW1Y 5DB England Tel: + 44 (0)20 74 51 73 00 Fax: + 44 (0)20 83 91 70 2 www.materials.org.uk
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